1. Field of the Invention
This invention relates generally to a method and apparatus for machining bone to fit an orthopedic surgical implant, and more particularly to a method and apparatus for machining a patella to fit a patellar implant.
2. Description of the Prior Art
The human knee joint sustains enormous stress even under ordinary circumstances, causing inevitable joint degeneration over time, and disease and aging only exacerbate this process. The surgical procedure termed "total knee arthroplasty" (colloquially known as knee replacement surgery) is therefore understandably common, and has been performed with increasing frequency in recent years.
The knee is formed where the femur meets the tibia. The head of the tibia includes two condyles that form two smooth concave surfaces which articulate with the condyles of the femur. The patella is a sesamoid (lens shaped) bone which slides in a groove located between the condyles of the femur and developed in the tendon of the quadriceps femoris muscle. The patella functions to increase the efficiency of the quadriceps muscle. As the knee articulates, the muscles and tendons force the patella toward the condyles of the femur.
Total knee arthroplasty replaces degenerate femoral condyles, and degenerate tibial plateau surfaces with prosthetic implants and, when necessary, the articular surface of the patella can also be replaced. When the latter is necessary, commonly an ultra-high molecular weight polyethylene articulating surface, with or without a metal baseplate, is implanted on the posterior side of the patella, adjacent the femoral condyles. This surgical procedure requires the knee to be approached through a longitudinal skin incision, followed by a medial parapatellar capsular incision. The quadriceps tendon is then incised longitudinally, allowing eversion and dislocation of the patella laterally exposing the articular surface of the patella.
Typically, guides or devices are used to assist the surgeon in removing the patella's articular surface. These devices include a plate adapted to engage an anterior surface of the patella opposite the articular surface. A template may then be positioned adjacent to the articular surface so that a cutting instrument, typically an oscillating saw may be guided over the articular surface forming a flat surface adapted to receive a patellar implant.
Patellar implants come in a variety of shapes and sizes, which are controlled by the manufacturer of the implant. The surgeon uses the patellar implants which are furnished with the particular total knee system that he/she has selected. The surgeon chooses the best size, and in some cases, has a choice of whether or not to use a metal-backed component. Every manufacturer supplies instrument and/or instructions for using generic instrumentation for preparing the patella surface to receive an implant. In preparing the surface on which the implant will rest or to which it will be cemented, a planar surface is usually desired. This is, with a few systems, a recessed planar surface in which case the implant is rounded rather than elliptical or irregular. This is the only type of system where the shape/size of the implant should correspond to the tooling used to produce the surface on which the implant rests.
The prior art guides severely limit the surgeon's freedom in deciding the shape of the patellar implant, since in many cases, the patellar implant manufacturer manufactures the corresponding guide for use in surgery. Unfortunately, the prior art guides can result in improperly and imprecisely oriented and shaped patellar surfaces adapted to receive the implant. This can result in an improper attachment of the implant to the remaining patella resulting in poor mechanics, and sometimes necessitating subsequent corrective surgery.
Some of the problems with prior art instrumentation are as follows:
1) There is no way of precisely determining orientation of the surface on which the patellar implant will rest; PA1 2) In many cases, there is no provision for adjusting the orientation of the patellar implant; PA1 3) There is an inability to cut a precisely planar surface; and PA1 4) There is an inability to produce a recessed surface which has other than a circular border.
Therefore, an object of the present invention is to eliminate those problems.
A brief summary of the history of joint replacement technology is instructive in illustrating the problems addressed in this specification. The first reliably successful major total joint replacement procedures were not performed until the late 1950s when prosthetic components were first implanted in bone with the use of methylmethacrylate as a grouting agent. More recently, prostheses have been introduced without grouting agents, and naturally the "press-fit" of such prosthetic implants must be more exacting to give successful results. Unfortunately, because the gap between bone and implant should not be more than about 0.005 inches, typical bone preparation methods including broaching, reaming and sawing usually result in gaps of typically 0.050 inches between prosthesis and bone. This has led to a standard practice of impacting slightly oversized prosthetic implants into bone sites to improve fit, but this in turn often causes bone fractures and even this method cannot eliminate all gaps.
It would be desirable, in view of the above, to be able to produce customized bone preparation sites to receive specific prosthetic implants. For example, it would be advantageous to produce a planar surface at the patellar and tibial surfaces of the knee, which planar surface is recessed slightly into the bone, so as to correspond with the outer border of the implant. This is not possible with an oscillating saw; as discussed previously, some patellar implants are available which have a circular cross section (which represents a compromise) to allow the use of a cylindrical reamer to provide the implantation site. Other implants require similar specialized bone site preparation, although techniques to achieve this have heretofore been unavailable.
Thus, a need remains for a method and apparatus for machining bone, such as the patella, to fit an orthopedic surgical implant that gives substantial freedom for the surgeon in defining (1) where to place the implant; (2) the orientation of the implant; and (3) how to machine the bone to result in a good fit between the bone and the implanted prosthesis.